difference between setting pose on rviz and pose on path

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Rviz show start pose and goal pose as following when I move a turtlebot with navigation.

ken@ken:~$ rosrun rviz rviz
[ INFO] [1405463918.673377718]: rviz version 1.10.14
[ INFO] [1405463918.673431850]: compiled against OGRE version 1.7.4 (Cthugha)
[ INFO] [1405463919.308185475]: Stereo is NOT SUPPORTED
[ INFO] [1405463919.308356676]: OpenGl version: 4.2 (GLSL 4.2).
[ INFO] [1405463939.325036819]: Setting pose: 1.642 0.173 0.000 [frame=map]
[ INFO] [1405463980.728412464]: Setting goal: Frame:map, Position(4.037, 1.068, 0.000), Orientation(0.000, 0.000, 0.000, 1.000) = Angle: 0.000 

After that, global_planner calculates path as following.

Info:    RRT: Created 2 states
Info:    Solution found in 0.003221 seconds
[ INFO ] [1405463978.828431527]: x(1.590040),y(0.194749),theta(0.006343)
[ INFO ] [1405463978.828567407]: x(4.036678),y(1.068085),theta(0.000000)
Info:    Path simplification took 0.000394 seconds
Geometric path with 2 states

The goal position of the result calculated by planner is same as one of Rviz, but the start position is different from by planner and Rviz. As the resolution is 0.05[m/pixel], the difference is only 1 pixel. Has Anyone experienced this situation? I want to know if the robot has move at that time. Actually, movement of my turtlebot is unstable. That means turtlebot sometimes moves, doesn't usually move. Thank you in advance!